1. Field of the Invention
This invention relates to an image read-out and reproducing apparatus for obtaining an image signal by reading out image information from a recording medium, on which the image information has been recorded, and reproducing a visible image by using the image signal.
2. Description of the Prior Art
Techniques for reading out image information recorded on a recording medium in order to obtain an image signal, carrying out appropriate image processing of the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value designed so as to match the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and by using the image signal the radiation image of the object is reproduced as a visible image on a recording material such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
In recent years, various attempts have been made to simplify the overall radiation image recording and reproducing system as much as possible, to make the apparatus smaller as a whole and to reduce the manufacturing cost thereof. As one of such attempts, a novel read-out and reproducing apparatus has been proposed in, for example, U.S. Patent Application No. 195,333, now U.S. Pat. No. 4,893,012. In the proposed read-out and reproducing apparatus, a read-out apparatus for obtaining an image signal by scanning a stimulable phosphor sheet, on which a radiation image of an object has been stored, with stimulating rays, and a reproducing apparatus for reproducing a visible image on a recording sheet by scanning the recording sheet with reproducing light, which has been modulated in accordance with the image signal, are combined into a single apparatus, so that both the image read out and the image reproduction is carried out by using a single scanning system.
More specifically, the read-out and reproducing apparatus proposed in U.S. Patent Application No. 195,333 is constituted so that a stimulable phosphor sheet, on which a radiation image of an object has been stored, is scanned in a main scanning direction with stimulating rays, which have been deflected by a light deflector, and is moved with respect to the stimulating rays in a sub-scanning direction which is approximately normal to the main scanning direction. When the stimulable phosphor sheet is exposed to the stimulating rays, it emits light carrying information about the stored radiation image. The emitted light is detected in order to obtain an image signal. A recording sheet is scanned in the main scanning direction with reproducing light, which has been modulated in accordance with the image signal and deflected by the light deflector, and is moved with respect to the reproducing light in the sub-scanning direction. In this manner, a visible image is reproduced on the recording sheet.
In the read-out and reproducing apparatus described above, scanning must be carried out by using stimulating rays having a predetermined amount in the course of image read out, and scanning must be carried out by using reproducing light which has been modulated in accordance with the image signal in the course of image reproduction. Accordingly, in the read-out and reproducing apparatus proposed in U.S. Patent Application No. 195,333, an acousto-optic device (hereinafter abbreviated to AOM) is inserted into the optical path of the light beam. In the course of image read out, no modulation is effected with the AOM, and main scanning is carried out by using a zero-order light component radiated from the AOM. In the course of image reproduction, main scanning is carried out by using a first-order light component which has been modulated by the AOM. This is because the power output of the zero-order light component cannot be made zero, and fog arises in a reproduced visible image when the image is reproduced by use of the zero-order light component. Therefore, the zero-order light component is not suitable as the reproducing light, and the first-order light component should be used as the reproducing light. On the other hand, in the course of the image read out, it is necessary to obtain stimulating rays having a predetermined amount and as large a power output as possible. However, the amount of the first-order light component is at the most approximately 90% of the amount of the light beam incident upon the AOM, and therefore the energy of the first-order light component is insufficient for stimulating rays.
However, the zero-order light component and the first-order light component are radiated in different directions from the AOM. Therefore, when deflection and main scanning are carried out without employing a particular means, the range over which main scanning is carried out with the zero-order light component and the range over which main scanning is carried out with the first-order light component deviate relative to each other. As a result, a visible image reproduced on the recording sheet shifts from the center portion of the recording sheet to an edge thereof, or part of the visible image is located on the outward side of the recording sheet and cannot be reproduced on the recording sheet. In order to eliminate these problems, the aforesaid read-out and reproducing apparatus is provided with an optical member, such as a mirror or a prism, which is positioned between the AOM and the light deflector. The optical member is moved between a position for image read out using the zero-order light component and a position for image reproduction using the first-order light component, so that the zero-order light component for image read out and the first-order light component for image reproduction impinge upon the light deflector along the same optical axis. In this manner, the range over which the stimulable phosphor sheet is scanned with the zero order light component in the course of image read out and the range over which the recording sheet is scanned with the first-order light component in the course of image reproduction are made to coincide with each other.
In cases where the optical member such as a mirror or a prism is moved between the position for image read out and the position for image reproduction as proposed in U.S. Patent Application No. 195,333, a mechanism for accurately moving the optical member is required, so that the scale of the read-out and reproducing apparatus becomes larger. Also, in the course of operating the accurate movement mechanism for a long period, position reproducibility of the optical member deteriorates and the optical axis thereof changes.